1. Field of the Invention
The present invention relates to a method and apparatus for polling transmission status in a wireless communications system, and more particularly, to a method and apparatus for preventing retransmission of PDUs that have been successfully transmitted, and reducing transmission latency.
2. Description of the Prior Art
The third generation (3G) mobile telecommunications system has adopted a Wideband Code Division Multiple Access (WCDMA) wireless air interface access method for a cellular network. WCDMA provides high frequency spectrum utilization, universal coverage, and high quality, high-speed multimedia data transmission. The WCDMA method also meets all kinds of QoS requirements simultaneously, providing diverse, flexible, two-way transmission services and better communication quality to reduce transmission interruption rates.
The access stratum of the 3G mobile telecommunications system comprises a radio resource control (RRC), radio link control (RLC), media access control (MAC), packet data convergence protocol (PDCP), broadcast/multicast control (BMC) and other sub-layers of different functions. The operations of the above-mentioned sub-layers are well known for those skilled in the art, and will not be further mentioned. In addition, the 3G mobile telecommunications system can provide different levels of transmission quality and operate in different modes according to the different requirements, for instance, Transparent Mode (TM), Unacknowledged Mode (UM), Acknowledged Mode (AM). TM is used for services requiring instant transmission, UM is used for services requiring instant transmission as well as packet orders, and AM is used for robust services requiring data accuracy but not instant transmission.
In AM, for better data accuracy and robustness, RLC layers of a transmitter and a receiver initiate a status report procedure at an appropriate time to have the transmitter poll the receiver so that the receiver transmits a status report, or a piggybacked status report to the transmitter. The transmitter then executes procedures followed according to the status report from the receiver. As the transmitter polls the receiver, the transmitter selects a Protocol Data Unit (PDU) and sets a polling bit of the PDU as ON (=1), which requests the receiver to transmit a status report. In such a situation, if there is no PDU scheduled for transmission or retransmission, the transmitter selects a PDU that has been transmitted, and sets a polling bit of the PDU to be 1 to poll the receiver. In other words, when a poll is triggered while no PDU is scheduled for transmission or retransmission, the transmitter shall select a PDU that has been transmitted to poll the receiver, causing unnecessary retransmission and waste of system resources.
In order to improve the afore-mentioned problem, the prior art provides a method for triggering a poll, where a POLL SUFI (Super Field) scheme is applied. That is, when the transmitter needs to poll the receiver, the transmitter can send a STATUS PDU containing a POLL SUFI to request a status report from the receiver. In the prior art, the POLL SUFI is composed of a Type field and a Poll_SN (Sequence Number) field. The Poll_SN field comprises 12 bits, and is configured by the following rule. When “Poll” is configured and the value of “Configured_Tx_Window_Size” is larger than or equal to “2048”, the Poll_SN field shall be set to VT(S)−1, where “Configured_Tx_Window_Size” represents the size of the transmission window, and VT(S) represents SN of the next PDU to be transmitted for the first time. Otherwise, if “Poll” is configured and the value of the “Configured_Tx_Window_Size” is less than “2048”, the Poll_SN field can either be set to VT(S)−1 or SN of a PDU that has not been discarded and has not yet been acknowledged by the peer entity. Accordingly, after receiving the STATUS PDU containing POLL SUFI, the receiver considers that a PDU with SN=Poll_SN has been transmitted by the transmitter, and transmits a status report corresponding to its receiving status to the transmitter.
Via POLL SUFI, the prior art can prevent unnecessary retransmission. However, the POLL SUFI method may cause transmission delay and degradation of efficiency. For example, suppose that the “Configured_Tx_Window_Size” is configured to be 1024, and PDUs of SN=0, 1, 2, 3 have been transmitted. In other words, VT(S)=4. Suppose that none of the four transmitted PDUs are positively acknowledged. When a poll is triggered, by the prior art, the transmitter can send a POLL SUFI containing Poll_SN equal to VT(S)−1 or SN of a PDU that has not been discarded and has not yet been acknowledged by the receiver. In other words, the transmitter can set Poll_SN to 0, 1, 2, or 3. If the transmitter sets Poll_SN to 0 while the receiver did not receive any of the four AMD PDUs before receiving the POLL SUFI, the receiver can only consider that the PDU of SN=0 has been transmitted, and send a status report to negatively acknowledge SN=0. The transmitter will retransmit the PDU of SN=0 with polling bit set. If the retransmitted PDU is received successfully, the receiver will positively acknowledge SN=0. In such a situation, the transmitter need to poll again with a POLL SUFI, to retransmits other PDUs. Thus, retransmission of SN=3 will be delayed for quite a long period of time. In other words, using the POLL SUFI to poll the receiving, the prior art method can prevent retransmission of transmitted PDU, but may cause transmission delay and degradation of efficiency.